1. Field of the Invention
The present invention relates to an information recording medium and a production process therefor.
2. Description of Related Art
Various kinds of optical recording media and magnetic recording media are known as information recording media having recording films capable of recording and storing information. Among such recording media, magneto-optical disks, phase change optical disks, floptical disks, hard disks and the like are known as rewritable information recording media. Information is recorded on these information recording media by forming a magnetic domain in their recording films by magnetic means including thermo-magnetic recording means or by forming a part where atomic or molecular orientation of the recording films is changed by thermal means. The recorded information is reproduced by detecting a turn of a plane of polarization by the magnetic domain, a change in reflectance or the like.
In the information recording media, recording and reproduction are carried out at a predetermined position by means of tracking.
For example, in the case of magneto-optic disks, guide grooves (referred to simply as grooves hereinafter) are formed on the disks. In magneto-optic disks currently available in the market, information is recorded in a recording film on the top of a projected portion (a land) between grooves. However, for higher recording density, it has been proposed to record and reproduce information both on lands and in grooves (a land/groove recording method).
In this method, since both the lands and the grooves are recording tracks, the spacing between the tracks is smaller. That may cause cross-writing and cross-erasing. The cross-writing means that information is also recorded on a track adjacent to a track on which the information is intended to be recorded, and the cross-erasing means that information is erased from a track adjacent to a track from which information is intended to be erased.
In order to prevent the cross-writing and cross-erasing, it has been proposed to enlarge the distance between the bottom of the grooves and the top of the lands, that is, to deepen the depth of the grooves. However, if the grooves are deepened, the recording film adheres to the bottom of the grooves in a smaller amount, and therefore, it is difficult to obtain a recording film with desired characteristics.
Japanese Unexamined Patent Publication No. HEI 10 (1998)-275369 reports a solution to the above-described problem. According to this publication, the grooves have a wall angle within the range of 70 to 90xc2x0 with respect to the bottom of the grooves, and the recording film is formed by a method of high directivity. It is reported that thereby the recording film on the top of the lands can be thermally and magnetically separated from the recording film at the bottom of the grooves.
Here, as shown in FIG. 1, the wall angle means an angle xcex1 defined by a bottom face g of a groove and a tangent line f to the wall at half the maximum height d of the groove (a level difference between the top of the land A and the bottom of the groove B) in a sectional view orthogonal to the direction of a recording track.
The mass-production of substrates is achieved by the following process. First, a metal transfer mold (what is called a stamper) is produced from an original disk on which grooves and pits are formed. An ultraviolet curing resin or a molten resin is cast onto the stamper and cured. The cured resin is peeled off the stamper to give a substrate.
However, a problem is that it is difficult to take the stamper off the original disk if the wall angle is large. This problem is significant when the wall angle is 80xc2x0 or larger and is further serious when the grooves are deep.
Furthermore, from the viewpoint of magnetic and thermal separation of adjacent tracks, the wall angle is preferably 90xc2x0 or greater, that is, the walls of the grooves are preferably in a so-called overhanging state. However, the production of such substrates involves the stamper engaging with the original disk and the substrate of cured resin engaging with the stamper in the form of dovetail joints. Therefore, it is physically impossible at the level of today""s technology to disengage them from each other.
Besides the above mentioned, it is known to form the grooves directly by writing means such as a laser. However, it is difficult to form grooves with a wall angle of 80xc2x0 or more by such means.
The present invention provides a process for producing an information recording medium comprising the steps of:
forming, on a substrate provided with a groove having a wall which forms an angle smaller than 80xc2x0 with respect to a bottom of the groove, an underlayer having a wall which forms an angle of 80xc2x0 or more with respect to the bottom of the groove, by a film forming method of a low directivity; and
forming at least a recording film on the formed underlayer at the bottom of the groove, on the top of a projected portion between grooves or both at the bottom of the groove and on the top of the projected portion.
The present invention also provides an information recording medium comprising:
a substrate with a groove formed thereon;
an underlayer formed on the substrate to cover at least a wall of the groove; and
a recording film formed at least at a bottom of the groove, on the top of a projected portion between grooves or both at the bottom of the groove and on the top of the projected portion,
wherein the wall of the groove has an angle smaller than 80xc2x0 with respect to the bottom of the groove and the wall of the underlayer has an angle of 80xc2x0 or more with respect to the bottom of the groove.
The present invention further provides an information recording medium comprising a substrate having a groove, an underlayer which has been formed on the substrate with rotating the substrate, and a recording film which has been formed on the underlayer with maintaining the substrate in a stationary state.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.